1. Field of the Invention
The invention relates to HSDPA (High Speed Downlink Packet Access) radio systems. More precisely, the invention relates to controlling link adaptation and packet scheduling in an HSDPA radio system.
2. Description of the Related Art
In the ongoing standardization work in WCDMA (Wideband Code Division Multiple Access) evolution in 3GPP (3rd Generation Partnership Project), a concept called HSDPA (high speed downlink packet access) has been introduced for Release 5. One of the central elements of this concept is a fast layer one scheduling combined with a hybrid automatic repeat request (H-ARQ). The fast layer one scheduling attempts to exploit the current channel conditions as well as possible to achieve high instantaneous throughput.
The layer one scheduling includes two essential parts: a link adaptation and a packet scheduling. The link adaptation in HSDPA is the ability to adapt the modulation scheme, the number of multicodes and forward error correction coding according to the quality of the radio link. The link adaptation determines the data rate to be used for the given channel conditions and the packet scheduling determines when a user is active on the HS-DSCH (High Speed-Downlink Shared Channel). Scheduling of the transmission of data packets over the air interface is performed in Node B (Base Transceiver station, BTS). The H-ARQ feature provides a mechanism to retransmit a packet if it is received erroneously. In HSDPA, the request for retransmission of the data packets is processed in the base station. For the scheduling unit to obtain knowledge of the instantaneous channel conditions, a feedback channel has been introduced from the user equipment to the Node B called HS-DPCCH (High Speed-Dedicated Physical Control Channel). This channel carries a channel quality indicator (CQI), which informs the Node B about the current downlink channel conditions.
Furthermore, this channel carries signalling to acknowledge whether a previously received packet has been detected correctly or not (H-ARQ ACK (positive acknowledgement)/NACK (negative acknowledgement) information). The H-ARQ information is used by the Node B to determine if a packet is to be retransmitted, and the CQI is used by the link adaptation to determine the modulation and coding schemes as well as the number of multicodes to be used for transmissions towards the user in question. The CQI measurements and the operation of the link adaptation mechanism can also be used to aid the decision making of the packet scheduler to improve utilization of the network resources. In some modes, the link adaptation mechanism can also be improved by employing measurements of the power used for the associated downlink DPCH (Dedicated Physical Channel) controlled by power control commands sent on from the user equipment to the Node B. However, when the user equipment is in soft handover (SHO), the power control will operate relative to more than one Node B, and consequently this measure cannot be used to estimate the channel conditions.
In order to provide a better uplink coverage, the HSDPA concept allows the repetition of the CQI and ACK/NACK information. However, from the perspective of link adaptation and packet scheduling, this causes extra delays in the link adaptation cycle, which again leads to a significantly decreased link and system performance in most cases. After the propagation to the Node B, a code word is estimated and assuming there are no delays in the Node B, a decision for transmission is taken for this user equipment. For example, if a repetition factor 3 is used for the HS-DPCCH and a ‘zero-delay’ assumption for the Node B processing, then the CQI value can be utilized after 12 slots, corresponding to 8 milliseconds.
The problem with this is that the CQI information gets ‘older’ with increased delays, and therefore contains less information about the current channel conditions. This is particularly a problem in the case of soft handover where the Node B can only rely on the CQI reports since the power measurements from the associated downlink DPCH will not give the information on the link quality between the Node B in charge of the HSDPA link and the user equipment. Thus, there is a need for an improved link adaptation performance in the Node B.